Diesel engines can provide high torque and improved fuel economy as compared to gasoline engines. However, the temperature of diesel exhaust gases is lower than that of many gasoline engines. Therefore, it may take longer for catalysts in an exhaust system of the diesel engine to reach a temperature where a threshold catalyst efficiency temperature is reached (e.g., a catalyst light off temperature). Further, many diesel engine are turbocharged to improve engine output, and exhaust gas heat is extracted by the turbocharger. Consequently, even less exhaust gas heat may be available to light off the catalyst. In view of exhaust gas heating challenges associated with diesel engines, it may be desirable to provide for a way to improve diesel engine starting.
The inventor herein has recognized the above-mentioned disadvantages and has developed an engine operating method, comprising: operating an engine at idle conditions with a first EGR percent concentration at an engine temperature less than a first engine temperature; and operating the engine at idle conditions with a second EGR percent concentration at an engine temperature greater than the first engine temperature, the second EGR percent concentration less than the first EGR percent concentration.
By operating an engine after a cold engine start with a higher level of exhaust gas recirculation (EGR) than is delivered during similar warm engine operating conditions, it may be possible to increase engine exhaust gas temperatures to reduce catalyst light off time. Further, increasing EGR and retarding combustion phasing (e.g., location of peak cylinder pressure) can increase engine temperature in a reduced amount of time. Consequently, engine emissions may be reduced. In one example, engine speed and load may be increased via a hybrid powertrain to allow higher levels of EGR at engine idle conditions. The higher levels of EGR at idle can retard combustion timing so as to direct more combustion heat to the engine and engine exhaust system. One way to achieve higher engine loads and increase tolerance to higher percentages of EGR in cylinder mixtures is to utilize an auxiliary motor of a hybrid vehicle. For example, an electric motor can be used to control engine speed and engine load during engine idle conditions.
The present description may provide several advantages. In particular, the approach may reduce catalyst light off time, thereby reducing engine emissions. Further, the approach reduce engine warm up time so as to reduce engine feed gas emissions. Further still, the approach can provide good vehicle drivability via ramping out EGR and combustion phase retard during conditions where engine torque demand increases.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.